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CHAPTER 8 Transactions and Versioning
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Figure 8-3: Concurrent insertions to different tables
Data definition operations on a single table lock out all other readers and
writers from that table. See “Locks for DDL operations” for details.
Transactions use committed data
Committed data results when a write transaction commits. Every transaction
uses the latest committed version of the database as of the time the transaction
begins. It uses that version until the transaction commits.
The time a transaction begins is called its Start Timestamp. The start timestamp
can be any time before the transaction's first read. Any insertions and deletions
the transaction makes are reflected in the snapshot. Thus, for the user executing
a transaction, the image in the snapshot changes whenever that transaction
writes data to the table, and then reads it again. For all other users, the image
remains static until their transaction commits.